Coloring stainless steel



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Regina L. Hornak, Baltimore, Md., and John J. Halbig, Middletown, Ohio, aasignors to Armco Steel Corporation, a corporation of Ohio No Drawing. Application May 5, 1948, Serial No. 25,307

Claims. (c a-5) This invention relates to colored stainless steel products and to a method of producing the same.

An object of our invention is that of provid-, ing a direct and reliable method for coloring stainless steel products which is simple to control in its operation and effect, giving any'of a number of colors in readily reproducible hue.

A further object of our invention is-the provision, in a simple, efficient and thoroughly practical manner, of chemically surface-colored stainless steel products which are..,decorative, ornamental and pleasing to the eye. and in which the colored portion is strong, tough and durable as well as corrosion-resistant.

A still further object of our invention is directed to the achievement of dense, wear-resistant, colored surface films on stainless steel which are chemical reaction products of-the metal, durable in color, and pleasing in appearance.

Other objects of the invention in part will be obvious and in part pointed out more fully hereinafter.

The invention, accordingly, consists in the several steps and the relation of each of the same t one or more of the others as described herein and in the products, the scope of the application of which is indicated in the following claims.

As conducive to a clearer understanding of certain features of our invention, it may be noted at this point that stainless steels when provided in the natural finish have a dull or satin-like appearance if unpolishe'd, or in the polished condition, achieved for example by electrolytic polishing treatment or b ufllng, take on a bright mirror-like metallic hue. These steels of course are high-alloy steels essentially containing about to chromium, and occasionally including other ingredients in small amounts for special purposes, with the remainder substantially all iron. On most occasions in the past, the stainless steels have been put into use in their natural finish, very effectively and satisfactorily serving their intended purpose in this condition. With increasing demands for products of varied appearance, however, the trend has been toward the addition of surface color to the steels, for such purposes as giving ornamental and decorative eifects.

The art of coloring stainless steels introduces a number of complex problems, many of these rooting from the fact that the steels by virtue of their corrosion-resistance and surface inertness do not readily respond, if at all, to many classes of coloring treatment where the treatavoided by resorting to the use of paints and other applied coatings, these merely being applied without calling for chemical reacton with the metal. The paint coatings, however, usually are not practical, one reason for this being that they do not tie in satisfactorily with the properties of the steel; further, they usually render it inconsistent to use stainless steel as distinguished from some relatively inexpensive basic product in coating treatment.

In the field of chemical reaction coating treatment of stainless steel, it has sometimes been the practice to immerse the metal in an aqueous bath of treating compounds which give a, hydroxide surface coating. So many of these coatings are characterized by softness or. chalkiness and rub off or wear away in a very short time, in addition to being adversely affected by moisture. A variety of these coatings are unsatisfactory because of poor color quality or instability of color. Often too, they crack, chip or scale of! when the metal is even slightly stressed by bending or by thermal expansion or contraction.

It will be appreciated that many of the treating compounds of the prior art are not conducive to a satisfactory control over operations for re producing a desired shade of color on the stainless steel.

Other coloring ingredients heretofore utilized oifer no appreciable latitude for variation of color.

There are some few processes in the prior art of coloring stainless steel which, in order to be operative at all, require intense heat ranging say from 700 F. to 1700 F. or more. Among these are processes which impart a heat-tint t the metal surface depending in color upon the particular temperature of treatment. Another type of process in this general category involves subjecting the steel to a molten salt reaction bath, heated to a high temperature, the particular temperature having a highly critical influence as to the exact color obtained. While many of these processes are valuable, especially where certain colors are desired in the steel, it still remains that the prevailing temperatures call for hightemperature equipment and introduce related problems of heating and operation.

An outstanding object of our invention, ac-

" cordingly, is the provision of a simple and eiiecment is of a chemical nature. To some extent, as

difliculties of this type have heretofore been tive process for chemically coloring stainless steel by reaction with the metal surface, the process employing readily available bath ingredients and giving any of a variety of durable colorings, these with easy control over conditions producing basic color and hue.

Referring now more particularly to the practice of our invention, we color a variety of the straight chromium stainless steels, by the chemical action of a hot aqueous bath containing a small quantity of phosphoric acid r n ing in amount from about 0.25% to by weight of solution. The solution preferably consists of phosphoric acid in the amount lust noted, and the remainder substantially all water. We usually give the coloring treatment by immersing the steel in the hot acid bath.

The treatment in phosphoric acid produces a colored, thin, translucent and usually somewhat iridescent film on the stainless steel surface. As to color achieved, this mually is anywhere from yellow to purple, especially from green to blue, the particular color and hue importantly depending upon the time period of exposure of the steel to the acid solution. The temperature and concentration of the acid bath to some extent aifect the depth, vividness or weakness of the particular color and produce some change in hue, but the major changes occur as a function of time. Sometimes a slight coloring of the steel is noticeable after a period of about minutes immersion time in the phosphoric acid solution. A full color, however, seldom is obtainable short of about minutes, as for example with the solution at boiling temperature. By prolonging the hot acid coloring treatment of the stainless steel, appreciable changes in color are had up to 20 hours, or more, immersion time. We prefer to maintain the bath at boiling temperature, but temperatures below that point, which also support an active, though perhaps not quite as active, chemical efiect of the solution, sometimes are employed if desired. The boiling solution, however, usually gives the advantage of substantially constant treatingtemperature, this being a temperature which is easy to control.

In most instances, the hue of the colored steel has a basic color aspect which may be described as pastel, light and semi-brilliant supplemented by undertones; a base color in a postal shade, for example, often conveys understones of red, green, yellow, or even violet. The sum effect is basic color plus slight iridescence which is very pleasing to the eye. Changes of sunlight orartificial light create slight variations, increase or decrease the undertones or cause the basic hue to become bright or relatively dark.

Among the many and varying uses of our coloring process is the production of coloredstainless steel products in such fields as architectural trim, furniture, fixtures, hardware, appliances, or fittings,- and illustratively for salon, theater, restaurant, home, or hotel decoration. More particularly, we color such stainless steel products as curtain rods, door knobs, escutcheons, hinges, towel racks, shower stalls, plumbing fixtures, personal jewelry, cigarette cases, compacts, bracelets, earrings, buckles, buttons, clasps, and the like. Still further examples are electrical fixtures, wall plates, lamp housings, and bases, and toasters and table ware. The colors achieved on our products are durable as from the standpoint of being resistant to fading and corrosion of the color film substance. Moreover, the film resists the wear of reasonable amounts of abrasion. The color film substance is dense and tightly adherent; it does not chip or peel, crack or spall even when the metal is bent or deformed.

Because of the translucency and extreme thinness of the films which we achieved, the finish of 4 the stainless steel which existed before treatment,

although covered-was not obscured or dulled. A

mechanically or electrolytically polished surface, for example, when treated in accordance with our coloring process, takes on color but retains a mirror-like quality. Likewise, a ground or sanded finish presents its typical appearance but in color. Satin finishes, unpolished or etched surfaces on the steel, though colored by our process, also retain they typical appearance. Thus,

for example, in creations embodying such prod-.

rods, wire, or thelike, for example containing about 11.5% to 13.5% chromium, up to 0.15% carbon, with small amounts of manganese and silicon, traces of sulphur and phosphorus, and the remainder substantially all iron, thus achieving desired articles or products of the steel, the surface of which is to be colored. Sometimes, to relieve stresses set up during fabrication, we

- subject the products to an annealing treatment.

For the elimination of oxide scale, dirt, grease, or the like which frequently accumulates on the metal surface, as during shaping and forming and heat treatment, we cleanse the metal preferably by pickling, such as in a 20% nitric acid, 1% hydrofluoric acid solution. After the stainless steel products are substantially clean, which usually is so often after a. quick dip, they are rinsed in clean water. In the pickled and rinsed clean condition, the stainless steel is especially susceptible to a satisfactory color finish. Before coloring, we sometimes polish the steel, while on other occasions we apply our coloring treatment to the unpolished steel.

Upon treating the 11.5% to 13.5% chromium stainless steel products by immersion in a boiling aqueous bath under operating conditions differing from product to product to the extent of phosphoric acid concentration used, somewhat different colors develop because of the particular concentration and especially in view of treating time. Some idea of the variety of the colors obtained with different times of immersion in several dilutions of the phosphoric acid are noted in the table below. In these instances, the depth of color corresponding to a given treating time increases somewhat with concentration of the acid as will be readily appreciated of the colors corresponding to 3 hours and 16 hours treatment noted in the table.

Eflect of certain aqueous phosphoric acid solutions on color of 11.5% to 13.5% chromium stainless cutlery steel; for diflerent time periods of immersion treatment.

Upon achieving desired color on the 11.5% to 13.5% chromium stainless steel products or articles referred to, we remove the colored steel from the phosphoric acid bath. Before use the colored metal preferably is rinsed in clean water 5 to free the surface of the treating acid.

Thus it will be seen that there is provided in this invention an art and products in which the various objects hereinbefore referred to, together with many highly practical advantages are successfully achieved. It will be seen that the products and articles are durable and corrosion-resistant, and that they have a permanent and attractive color finish. Moreover, it will be seen that the process is simple to set up and control to the end of producing a desired color or reproducing the color or hue.

It will be appreciated that in certain instances the colored stainless steel articles and products which we produce are given a grinding or cutting treatment at localized areas to expose the steel in natural color, this for contrast with the color achieved by phosphoric acid treatment. The cutting illustratively gives initials or some other design in contrast. Other contrasts, such as between colors or hues, sometimes are eifected by treating the exposed stainless steel areas and also the adjacent colored areas, in one of our phosphoric acid baths, and continuing the treatment until the exposed steel takes on color or hue differing from that of the adjacent surface area.

As many possible embodiments may be made of our invention and as many changes may be made in the embodiment hereinbefore set forth, it is to be understood that all matter described herein is to be interpreted as illustrative and not as a limitation.

We claim:

1. In coloring stainless steel essentially containing about to 35% chromium, the art of producing a thin, colorful translucent surface film thereon, which comprises treating the steel, by immersion for at least about 20 minutes time, in a hot aqueous coloring solution essentially consisting of about 0.25% up to about 5% phosphoric acid by weight and the remainder substantially all water.

2. In coloring stainless steel essentially containing about 10% to 35% chromium, the art of producing a thin, adherent surface film thereon Hanging from yellow to purple, which comprises subjecting the steel to the chemical coloring action of a hot solution essentially consisting of phosphoric acid in amounts from about 0.25% to 5% by weight and the remainder substantially all water.

3. In coloring stainless steel essentially containing about 10% to 35% chromium, the art of producing a thin, adherent surface iilm thereon ranging in color from yellow to purple, which comprises immersing the steel in a solution essentially consisting of about 0.25% up to about 5% phosphoric acid by weight and the remainder substantially all water, with said solution at approximately boiling temperature, and employing a period of immersion ranging upward from about 20 minutes time to more than about 20 hours depending upon the color and hue desired.

4. A stainless steel product or article essentially containing about 10% to 35% chromium and having a translucent, somewhat iridescent, adherent surface color film had by immersion in a hot aqueous solution essentially consisting of about 0.25% to 5% phosphoric acid with remainder water.

5. A stainless steel product or article containing about 11.5% to 13.5% chromium and having a thin, translucent, slightly iridescent surface color film ranging in basic color from yellow to purple had by immersion in a hot aqueous solution essentially consisting of about 0.25% to 5% phosphoric acid with remainder water.

REGINA L. HORNAK. JOHN J. HALBIG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,059,783 Farnwortn Nov. 3, 1936 2,172,422 Uhlig Sept. 12, 1939 2,426,445 Frisch Aug. 26, 1947 OTHER REFERENCES Industrial and Engineering Chemistry, February 1931, pages -150.

Thum, Stainless Steel, 2nd edition, 1935, pages 173-174. 

3. IN COLORING STAINLESS STEEL ESSENTIALLY CONTAINING ABOUT 10% TO 35% CHROMIUM, THE ART OF PRODUCTING A THIN, ADHERENT SURFACE FILM THEREON RANGING IN COLOR FROM YELLOW TO PURPLE, WHICH COMPRISES IMMERSING THE STEEL IN A SOLUTION ESSENTIALLY CONSISTING OF ABOUT 0.25% UP TO ABOUT 5% PHOSPHORIC ACID BY WEIGHT AND THE REMAINDER SUBSTANTIALLY ALL WATER, WITH SAID SOLUTION AT APPROXIMATELY BOILING TEMPERATURE, AND EMPLOYING A PERIOD OF IMMERSION RANGING UPWARD FROM ABOUT 20 MINUTES TIME TO MORE THAN ABOUT 20 HOURS DEPENDING UPON THE COLOR AND HUE DESIRED. 